Not limited to, but particularly in the case where an electromagnetic beam is utilized to investigate a sample system which presents with a varying depth surface topology, it is important to provide an electromagnetic beam of a known lateral dimension and which presents with a relatively simple cross-sectional Intensity profile.
It is noted that often electromagnetic beams present with a substantially arbitrary intensity profile, with the highest intensity generally being located centrally, and with intensity decreasing with increasing radius. While such a beam intensity profile is typically acceptable for use in ellipsometry and related practices, it has been found that once the intensity of a substantially arbitrary profile beam of electromagnetic radiation has dropped to, as an arbitrary example, say ten (10%) of its maximum, that said intensity in many beams does not, always continue to decay directly to essentially zero (0.0). Instead, it often presents irregularly as a function of radius, (eg. easily visualized as being generally similar to the Fourier transform of a square wave). The cause of said irregular intensity profile can include such as optical element wavelength dependent diffraction, surface roughness or other non-idealities, and where, for instance, electromagnetic radiation is provided via an aperture or via the end of a light fiber contained in a cladding, such that electromagnetic radiation falls outside a geometric image thereof.
It would be of benefit, as regards obtaining accurate data from application of ellipsometers and the like systems, if the intensity of an electromagnetic beam could be forced to decay quickly to zero (0.0), rather than demonstrate an irregular intensity profile as a function of radius.
With an eye to the present invention, a Search of Patents was conducted. Perhaps the most relevant Patent identified is U.S. Pat. No. 5,517,312 to Finarov. Said 312 Patent describes application of a scattered light reducing system at the entry to a Detector of an Ellipsometer or Spectrophotometer System, which scattered light reducing system consists of two lenses with a pin-hole containing diaphram located midway therebetween, and at the focal lengths of said lenses. Said scattered light reducing system is present after a sample system and processes electromagnetic radiation after it interacts with said sample system. The pinhole is described as serving to provide high spatial resolution as well as reduce scattered light. Another Patent identified is that to Campbell et al., U.S. Pat. No. 5,148,323. Said 323 Patent describes a Spatial Filter in which a pinhole is located other than at the focal length of a converging lens. U.S. Pat. No. 3,905,675 to McCraken describes a Spatial Filter containing system which enables observation of a weak source of electromagnetic radiation in the presence of strong sources thereof. U.S. Pat. No. 5,684,642 to Zumoto et al., describes an optical transmission system for use in fashioning an electromagnetic beam for use in machining materials which combines a Spatial Filter and an Optical Fiber. U.S. Pat. No. 4,877,960 is identified as it describes masking energy from outside the target area of a in a microscope having dual remote image masking.
Patents identified in a Search specifically focused on the use of lenses, preferably achromatic, in ellipsometry and related systems are:                U.S. Pat. Nos. 5,877,859 and 5,798,837 to Aspnes et al.;        U.S. Pat. No. 5,333,052 to Finarov;        U.S. Pat. No. 5,608,526 to Piwonka-Corle et al.;        U.S. Pat. No. 5,793,480 to Lacy et al.;        U.S. Pat. Nos. 4,636,075 and 4,893,932 to Knollenberg; and        U.S. Pat. No. 4,668,860 to Anthon.        
The most relevant Patent found is U.S. Pat. No. 5,917,594 to Norton. However, the system disclosed therein utilizes a spherical mirror to focus an electromagnetic beam onto the surface of a sample in the form of a small spot. Said system further develops both reflection and transmission signals via application of reflective means and of reflection and transmission detectors. The somewhat relevant aspect of the 594 Patent system is that a positive lens and a negative meniscus lens are combined and placed into the pathway of the electromagnetic beam prior to its reflection from a focusing spherical mirror. The purpose of doing so is to make the optical system, as a whole, essentially achromatic in the visible wavelength range, and even into the ultraviolet wavelength range. It is further stated that the power of the combined positive lens and negative meniscus lens is preferably zero. It is noted that, as described elsewhere in this Specification, said 594 Patent lens structure, positioning in the 594 Patent system, and purpose thereof are quite distinct from the present invention lens structure and application to focus a beam of electromagnetic radiation. In particular, note that the 594 Patent lens is not applied to directly focus and/or recollimate a beam of electromagnetic radiation onto a sample system, as do the lenses in the present invention. And, while the present invention could utilize a meniscus lens in an embodiment thereof, the 594 Patent specifically requires and employs a negative meniscus lens to correct for spherical aberrations caused by off-axis reflection from a spherical mirror, in combination with a positive lens to correct for achromatic aberration introduced by said negative meniscus lens. Further, the present invention system does not require reflection means be present in the path of an electromagnetic beam after its passage through the focusing lens thereof and prior to interacting with a sample system, as does the system in the 594 Patent wherein a focusing spherical mirror is functionally required.
Various papers were also identified as possibly pertinent, and are:
A paper by Johs, titled “Regression Calibration Method for Rotating Element Ellipsometers”, Thin Solid Films, 234 (1993) is also disclosed as it describes a mathematical regression based approach to calibrating ellipsometer systems.
A paper by Nijs & Silfhout, titled “Systematic and Ramdom Errors in Rotating-Analyzer Ellipsometry”, J. Opt. Soc. Am. A., Vol. 5, No. 6, (June 1988), describes a first order mathematical correction factor approach to accounting for window effects in Rotating Analyzer ellipsometers.
A paper by Kleim et al, titled “Systematic Errors in Rotating-Compensator ellipsometry”, J. Opt. Soc. Am., Vol 11, No. 9, (September 1994) describes first order corrections for imperfections in windows and compensators in Rotating Compensator ellipsometers.
Other papers of interest in the area by Azzam & Bashara include one titled “Unified Analysis of Ellipsometry Errors Due to Imperfect Components Cell-Window Birefringence, and Incorrect Azimuth Angles”, J. of the Opt. Soc. Am., Vol 61, No. 5, (May 1971); and one titled “Analysis of Systematic Errors in Rotating-Analyzer Ellipsometers”, J. of the Opt. Soc. Am., Vol. 64, No. 11, (November 1974).
Another paper by Straaher et al, titled “The Influence of Cell Window Imperfections on the Calibration and Measured Data of Two Types of Rotating Analyzer Ellipsometers”, Surface Sci., North Holland, 96, (1980), describes a graphical method for determining a plane of incidence in the presence of windows with small retardation.
A paper by Jones titled “A New Calculus For The Treatment Of Optical Systems”, J.O.S.A., Vol. 31, (July 1941), is also identified as it describes the characterizing of multiple lens elements which separately demonstrate birefringence, as a single lens, (which can demonstrate reduced birefringence).
Finally, a paper which is co-authored by inventors herein is titled “In Situ Multi-Wavelength Ellipsometric Control of Thickness and Composition of Bragg Reflector Structures”, by Herzinger, Johs, Reich, Carpenter & Van Hove, Mat. Res. Soc. Symp. Proc., Vol. 406, (1996) is also disclosed.
Even in view of the known art, especially in the context of ellipsometer and spectrophotometer systems, a need exists for a means to fashion a beam with a radially arbitrary intensity profile that does not quickly decay to zero, into a beam in which the intensity relatively directly radially approaches zero intensity.